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What are the approximate age ranges these words refer to? For example, could you call an 11 year old a niño, a muchacho, a joven? I'm sure this varies from person to person and place to place, but in general what group of children do each of these words refer to?

Where I am in Mexico, joven seems to be often used very much like señorita. That is to say, it is often used to refer to an unmarried man, of practically any age, or when the marital status of a man younger than, say, mid-40s, is unknown.
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Flimzy♦Nov 22 '11 at 2:09

Must of the time It depends on your age. If you're an 80 years old person you could call a 25 person a "niño" and there's no problem.
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razpeitiaNov 23 '11 at 5:15

"Chaval" and "chavala" are very used here in Spain, specially for teenagers (under 20) and sometimes for young adults (under 30).
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rsuarezNov 23 '11 at 8:44

@rsuarez Note that the British English word chav (a word unknown in North America), which seems cognate to the commonly heard chaval/chavala of Spain, has a despectivo or pejorative aspect in English that chaval doesn’t have in Spanish.
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tchristFeb 27 '12 at 3:49

I'm not sure I've heard "joven" and "mozo/moza" in Spain in any other than humourous context. They're a bit dated, in my opinion. We use "moza" in galician to refer to one's girlfriend or to young girls in general (look for "Domingo das Mozas" here: lugoturismo.com/fiestas/?idioma=i&pag=interesnacional), but it's pretty regional.
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rsuarezNov 23 '11 at 8:48

In Argentina "niño" is a little formal, "nene/a" is more used.
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leonbloyJan 25 '13 at 23:15

Throughout Central America joven is often used as a polite way to get the attention of any working server (no matter their age, really.. even old people), such as a waiter or waitress or someone working in a toy store.

Muchacha is a weird one in Guatemala at least, and I believe at least Honduras and El Salvador too. It's usually used to describe a servant or maid of the household. Not when addressing her (we addressed them by name), but when referring to them in general.

Mi cuñada está buscando muchacha, pero cuesta encontrar estos días.

Muchacho is seldom used, if at all. The equivalent would be something like patojo. Which I've never heard outside of Guatemala to be honest.

There's also nene/nena, usually when referring to smaller children, but also when talking with affection towards a person, mostly younger one.

This is the same case in México, "muchacha" o reducido a "chacha" significa "sirvienta", "empleada doméstica" y "criada". So you have to use "chava", "señorita" or "jovensita" for young adult women.
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user478249Jan 18 '14 at 15:33